In the past, analog signal processing circuits have been provided by discrete components. In such arrangements the topology of the circuit and the values of the components to be chosen at the outset have to be "correct" for the particular application. Such circuits have a disadvantage that where changes in circuit requirements occur, it may be difficult to reconfigure the circuit to have a different topology.
More recently, analog integrated circuits have been provided. Such circuits aim to include many or all of the required integers within the integrated circuit. However, the cost of making an integrated circuit for each different possible application would be prohibitive, and accordingly it has become known to provide multi-purpose analog integrated circuits otherwise known as integrated circuit analog systems which have circuit portions which can be selectively enabled or disabled to customize the circuit for a particular application, and which have circuit elements which can be programmed to provide, for example, a desired impedance.